Adapt testing suite to new open system (devel branch)

In the `devel` branch we have recently introduced a more powerful way to include perk files. Assume the following is `main.perk`

```
open "example.perk"
....
```

When compiling `main.perk` the new include system generates two files `main.c` and `example.c`. Currently, this breaks the test suite, because the testing script assumes that each `.perk` file generates *only one* `.c` file.